1. Field of the Invention
The present invention generally relates to an external storage system or equipment and more particularly to an external storage system typified by a magnetic disk system for an information processing system such as an OLTP (OnLine Transaction Process) system, a RDB (Relational DataBase) system and the like for which a high throughput and a high-speed I/O operation are required.
2. Description of the Related Art
With the advent of high-performance microcomputers, there have been developed computer systems which are capable of executing data processing at an extremely high speed. Besides, in accompanying a trend for implementation of databases or the like in a large capacity, the amount or scale of data handled by these computer systems tends to increase more and more.
Under the circumstances, the number of I/O requests issued per unit time by the computer system to a magnetic disk subsystem, which is a typical one of the external storage systems, has increased dramatically in recent years. Unfortunately, the access speed which the magnetic disk subsystem allows is extremely low when compared with the operation speed of a CPU (Central Processing Unit) incorporated in the computer as well as the access speed of a main memory because the access to the magnetic disk subsystem intrinsically involves mechanical operations. For this reason, the magnetic disk subsystem provides a bottleneck in enhancing the processing capability of the computer system. Put another way, the performance of the computer system is affected remarkably by the throughput of the magnetic disk subsystem.
Heretofore, as an attempt to solve the problem mentioned above, it is known to provide a read cache operative on the LRU (Least Recently Used) data principle for the external storage system such as a magnetic storage subsystem. More specifically, data read out from a magnetic disk is once placed in the read cache in accordance with the LRU principle, and when the host system issues a read request, it is checked in the external storage system itself whether or not the data requested by the host system is resident in the read cache. When the data of concern is found in the cache memory (i.e., upon cache hit), the data in the cache memory is transferred to the host system without making access to the external storage system such as the magnetic disk subsystem. In this manner, the time required for reading the data from a magnetic disk medium can be shortened.
In this regard, there is disclosed in JP-A-60-74057 a technique tackling the solution of a so-called device cross call problem by connecting a plurality of disk controllers to a plurality of magnetic disk drivers. This technique is also based on application of the LRU read cache scheme.
The techniques mentioned above can not be applied to the data write operation for the magnetic disk at all. Consequently, it is impossible to shorten the time taken for writing data in the magnetic disk. For these reasons, it can be said that the I/O performance of the magnetic disk subsystem can not always satisfy the requirements imposed by the computer system, when viewed in total.
Further, in the external storage system known heretofore, the magnetic disk controller incorporated therein is designed to issue in response to a read/write command generated by a host system a seek command to a disk drive of hierarchically lower rank. In that case, the interface bus is retained in the connected state during a seek period or time taken for a magnetic head to reach a track of concern and a sector waiting time taken for a relevant sector arrives at a position beneath the head. Consequently, in case the I/O requests are issued by the host at an extremely high frequency, the interface bus provides a bottleneck which incurs a problem that the number of I/O requests which can be processed by the external storage system (e.g. magnetic disk subsystem) is thereby limited or restricted.
In recent years, in view of the requirement for a large storage capacity of the external storage system of the computer as well as dispersion of processings as in the case of a client/server system, it has been attempted to increase the capacity of the external storage system by incorporating a number of magnetic disk drives in a magnetic disk subsystem while connecting a plurality of host computers to the magnetic disk subsystem. Thus, the latter has to process the access requests issued from the plurality of host computers. In that case, the controller incorporated in the magnetic disk subsystem has to control interconnections between a plurality of host computers and a plurality of magnetic disk drives.
According to the known technique mentioned above, the controller is connected to plural host computers and plural magnetic disk drives in a daisy-chain configuration. When an I/O request is issued from a given one of the computers, the interface bus is occupied by that given computer until the I/O request issued by the same has been disposed of. In the meanwhile, the other hosts or computers are forced to wait for release of the bus in the standby state. In this manner, the interface bus provides a bottleneck in that the number of the I/O requests allowable to be issued simultaneously is restricted. This problem may be solved by providing the interface bus for each of the host computers or the disk drives. In that case, however, the controller will become intolerably expensive and of very large scale, giving rise to another problem.